Localization of a sound signal in the vertical plane under masking conditions

The effect of the masker on the localization of the moving signal was investigated in the free field conditions. The experiments were carried out in an anechoic chamber. Sound signals were presented from loudspeakers located on a semicircular arc in the horizontal plane. Bandpass noise bursts (5–18 kHz) were used to create a signal and a masker. The signal and the mask were uncorrelated stimuli and were created from two independent noise bursts. The stationary masker was always on the right at an angle of 15°. The moving signals traveled to or from the masker along two paths located at two places (–86° to –52° and –52° to –18°). The signal and the masker of 1-s duration each were presented either simultaneously or with a delay of the signal onset relative to the masker onset. The delay varied from 1 to 40 ms and 1200 ms. The subjects evaluated the start and end points of the trajectory of the moving sounds. Localization data for a moving signal under masking conditions were compared with spatial estimates of the same signal when presented in isolation (without a masker). Localization of the start and end points of the signal in masking condition was compared with localization of the moving source alone. Results showed that the masker affected the start and end points of the signal trajectory. The shift depended on the direction of movement. The starting points were always shifted in the direction of motion of the signal. The end points were shifted in the opposite direction.

The effect of the masker on the localization of the signal was investigated in the free field conditions. Bandpass noise bursts (5–18 kHz) were used to create a signal and a masker. In the case of correlated stimuli, the same noise burst served both as a masker and a signal. In the case of uncorrelated stimuli, the signal and the masker were created from two different noise bursts. The masker was always on the right at an angle of 15 degrees. The signal was presented in one of three positions on the left: –18, –52, –86 degrees. The signal and the masker of 1 s duration each were presented either simultaneously or with a shift of the signal onset relative to the masker onset. The delay varied from 1 to 1200 ms. Perceived position of signals under masking conditions were compared with a single presentation of the signal. It is shown that under the masking conditions the perceived position of the signal shifted towards the masker, and the perceived position of the masker shifted towards the signal. The shift value decreased with increasing delay between the signal and the masker and with increasing angular distance between them. The mutual influence of the signal and the masker was more pronounced for correlated stimuli than for uncorrelated ones.

The effect of the masker on the localization of the moving signal was investigated in the free field conditions. Bandpass noise bursts (5–18 kHz) were used to create a signal and a masker. The signal and the mask were uncorrelated stimuli and were created from two independent noise bursts. The stationary masker was always on the right at an angle of 15 degrees. The moving signals traveled to or from the masker along two paths located at two places (–86°…–52° and –52°…–18°). The signal and the masker of 1 s duration each were presented either simultaneously or with a delay of the signal onset relative to the masker onset. The delay varied from 1 to 40 ms and 1200 ms. The subjects localized the start and end points of the trajectory of the moving sounds. Localization of the start and end points of the signal in masking condition was compared with localization of the moving source alone. Results showed that the masker affected the start and end points of the signal trajectory. The shift depended on the direction of movement. The starting points were always shifted in the direction of motion of the signal. The end points were shifted in the opposite the direction.

The precedence effect in the localization of a moving lagging sound source was studied in experiments on humans under the free field conditions in the presence of a stationary (lead) sound source. Broad-band noise (5–18 kHz) bursts 1 s in duration presented in the horizontal and vertical planes were used as signals. The lead-lag delays ranged from 1 to 40 ms. The results showed that, if the signals were presented in the horizontal plane, the probability of correct localization of the moving lagging signal was decreased for delays shorter than 25 ms; if the signals were presented in the vertical plane, it was decreased for delays shorter than 40 ms. If the delays were shorter than 8–10 ms, the subjects could not localize the moving lagging signal at all. In this interval of delays, the subjects could localize only the lead signal. The mean echo threshold for signals presented in the horizontal plane was smaller than for signals presented in the vertical plane (7.3 and 10.1 ms, respectively). However, comparison of these values across the sample of subject did not show significant differences [F(1, 5) = 5.52, p = 0.07]. The results of the study suggest that the precedence effect causes a tendency towards a stronger suppression of a moving lagging signal in the vertical plane than in the horizontal plane.

Monaural masked thresholds for clicks were established on three listeners. Masking functions across selected Δt's (the time disparities between the masker and click) were determined for four masking conditions in which the 70 dB SPL masker was a 500-msec burst of random noise. These four conditions were (1) simultaneous masking, (2) forward masking (FM) at Δt's from 1 to 250 msec, (3) backward masking (BM) at Δt's from −1 to −250 msec, and (4) forward-backward masking at selected Δt's between two 500-msec noise bursts. In this last instance, the interval between bursts was varied to achieve seven interburst intervals (IBIs) ranging from 25 to 500 msec. Simultaneous masking was 3–5 dB less at the boundaries of the noise burst than it was in the middle of the noise burst. The functions for FM and for BM were not inversely symmetrical with one another, although both showed less threshold shift (TS) the further displaced in time the click was from the masker. The masking that appeared when the click was presented between two noise bursts was greater immediately adjacent to a noise burst than near the center of the IBI. During the longer IBIs (500–300 msec), the configuration of masking was equivalent to that of FM followed by BM, with slight excess appearing only in the interval 20–40 msec before the second noise burst. The configurations of FM and BM were preserved during the shorter IBIs (200–50 msec), but the TSs were greater than either type would have produced if operating alone. These TSs became larger as the IBI was shortened, so that the brief IBIs were permeated with large amounts of masking even in the middle of the IBI. This cumulation of masking was so great when the IBI was reduced to 25 msec that the masking function lost the configuration characterizing the longer IBIs. The TSs during shorter IBIs exceeded the sum of the residual effective powers of FM and BM.

The ability of Long-Evans hooded rats (n=10) to detect sounds presented from sources in the horizontal plane at 0° elevation and the effects of bilateral lesions of the inferior colliculus on these abilities were examined. Rats were trained on a directional detection task which required animals to suppress licking responses in a conditioned avoidance paradigm when 100-ms noise bursts were presented at random from speakers at 45° intervals beginning at azimuth (0°). A task performance rate was determined by reducing the correct lick suppression rate for signal trials by the proportion of incorrect suppression responses on non-signal trials. Higher performance rates were observed for stimuli presented from 0-90° than for stimuli presented in the caudal hemifield prior to surgical procedures. Bilateral lesions restricted to the inferior colliculus reduced detection performance (p<0.05) and shifted the best performance rates from sounds presented at 0-45° to stimuli emitted from a 90° source (p<0.05). These results demonstrate that pigmented rats show differential detection levels for noise bursts presented from different locations throughout the horizontal interaural plane, and suggest that the inferior colliculus is involved in this aspect of directional hearing.

The present study examined cortical parallels to psychophysical signal detection and sound localization in the presence of background noise. The activity of single units or of small clusters of units was recorded in cortical area A2 of chloralose-anesthetized cats. Signals were 80-ms click trains that varied in location in the horizontal plane around the animal. Maskers were continuous broadband noises. In the focal masker condition, a single masker source was tested at various azimuths. In the diffuse masker condition, uncorrelated noise was presented from two speakers at +/-90 degrees lateral to the animal. For about 2/3 of units ("type A"), the presence of the masker generally reduced neural sensitivity to signals, and the effects of the masker depended on the relative locations of signal and masker sources. For the remaining 1/3 of units ("type B"), the masker reduced spike rates at low signal levels but often augmented spike rates at higher signal levels. Increases in spike rates of type B units were most common for signal sources in front of the ear contralateral to the recording site but tended to be independent of masker source location. For type A units, masker effects could be modeled as a shift toward higher levels of spike-rate- and spike-latency-versus-level functions. For a focal masker, the shift size decreased with increasing separation of signal and masker. That result resembled psychophysical spatial unmasking, i.e., improved signal detection by spatial separation of the signal from the noise source. For the diffuse masker condition, the shift size generally was constant across signal locations. For type A units, we examined the effects of maskers on cortical signaling of sound-source location, using an artificial-neural-network (ANN) algorithm. First, an ANN was trained to estimate the signal location in the quiet condition by recognizing the spike patterns of single units. Then we tested ANN responses for spike patterns recorded under various masker conditions. Addition of a masker generally altered spike patterns and disrupted ANN identification of signal location. That disruption was smaller, however, for signal and masker configurations in which the masker did not severely reduce units' spike rates. That result compared well with the psychophysical observation that listeners maintain good localization performance as long as signals are clearly audible.

Multi-channel cochlear implants have enabled many patients with severe-toprofound hearing loss to achieve near-normal levels of communication in some quiet listening situations (Helms, Muller, and Schon 1997). Bilateral implantation, which has been increasingly performed in recent years, affords the additional potential advantage of an increased awareness of auditory space. Initial studies of persons with bilateral implants have reported that localization of sources in the horizontal plane is superior when both implants are active than when either implant is turned off (Tyler, Gantz and Rubinstein 2002; van Hoesel, Ramsden, and O’Driscoll 2002; Nopp, Schleich, and D'Haese 2003). The superior performance under bilateral-implant conditions suggests that subjects are able to take advantage of interaural differences produced by their cochlear implant devices. The present study extends previous work by testing horizontal-plane localization performance in a large number of bilaterally-implanted persons (eventually 24), employing a large number of sound sources spanning the frontal horizontal plane. In addition, in order to determine if a speech stimulus may be more readily localizable than a noise burst, the current investigation measured localization performance with both types of signals.

The effects of burst duration and stimulus onset asynchrony (SOA, the onset–onset time difference) on the minimum audible angle (MAA) were measured in the horizontal and vertical planes using high-pass noise bursts. Four listeners were tested with two burst durations (10 and 50 ms) and five SOAs (25, 50, 100, 200, and 400 ms), using an adaptive paradigm. In both planes, MAAs were lowest at burst duration=50 ms, and the MAAs decreased exponentially with SOA. Although the effect of burst duration was generally larger in the vertical plane than in the horizontal plane, the plane of presentation did not affect the relationship between SOA and MAA.

Sound Signal Localization in Conditions of Masking in the Vertical Plane

Future touch screen applications will include multiple tactile stimuli displayed simultaneously or consecutively to single finger or multiple fingers. These applications should be designed by considering human tactile masking mechanism since it is known that presenting one stimulus may interfere with the perception of the other. In this study, we investigate the effect of masking on the tactile perception of electrovibration displayed on touch screens. Through conducting psychophysical experiments with nine participants, we measured the masked thresholds of sinusoidal electrovibration bursts (125 Hz) under two masking conditions: simultaneous and pedestal. The masking signals were noise bursts, applied at five different sensation levels varying from 2 to 22 dB SL, also presented by electrovibration. For each participant, the thresholds were elevated as linear functions of masking levels for both masking types. We observed that the masking effectiveness was larger with pedestal masking than simultaneous masking. Moreover, in order to investigate the effect of tactile masking on our haptic perception of edge sharpness, we compared the perceived sharpness of edges separating two textured regions displayed with and without various types of masking stimuli. Our results suggest that sharpness perception depends on the local contrast between background and foreground stimuli, which varies as a function of masking amplitude and activation levels of frequency-dependent psychophysical channels.

In this chapter, we investigated the effect of masking on the tactile perception of electrovibration displayed on touch screens. Through conducting psychophysical experiments with nine subjects, we measured the masked thresholds of sinusoidal electrovibration bursts (125 Hz) under two masking conditions: simultaneous and pedestal. The masking stimuli were noise bursts, applied at five different sensation levels varying from 2 to 22 dB SL, also presented by electrovibration. For each subject, the detection thresholds were elevated as linear functions of masking levels for both masking types. We observed that the masking effectiveness was larger with pedestal masking than simultaneous masking. Moreover, in order to investigate the effect of tactile masking on our haptic perception of edge sharpness, we compared the perceived sharpness of edges separating two textured regions displayed with and without various types of masking stimuli. Our results suggest that sharpness perception depends on the local contrast between background and foreground stimuli, which varies as a function of masking amplitude and activation levels of frequency-dependent psychophysical channels.KeywordsMaskingPsychophysicsTactileTouchscreenElectrovibrationVirtual edgeSharpness

Discriminability of bursts of reproducible noise was measured using a same‐different psychophysical method. Bursts in a pair were identical on “same” trials. On “different” trials, bursts were identical except for τ ms of independent noise located in the middle of the burst pairs. Three burst durations were examined: 1, 10, and 100 ms. Discriminability increased as the duration of the noise bursts increased. Randomly varying the level of each burst of noise impaired performance at all durations. Further experiments examined whether discrimination was based on spectral or energy cues. On each “different” trial, the order of the samples in the first digitized burst was randomized to generate the second burst in a pair. Results showed that, with the exception of the 1‐ms bursts of noise, discrimination was based the spectral shape of the waveform. [Work supported by NIH and AFOSR.]

The results of studying the precedence effect in the case where the direct and delayed (reflected) signals are located in the vertical and horizontal planes are considered. Loudspeakers emitting direct and reflected sounds were placed 45 deg to the left and right of the median line of the subject’s head in the horizontal plane and in front of and above the subject’s head, i.e., with 0 and 90 deg of elevation relative to the eye level, in the vertical plane. It has been shown that the time limits of the precedence effect of short (5-ms) signals are similar in the horizontal and vertical planes. For signals more than 10 ms in duration, the values of echo thresholds were higher in the vertical plane and significantly differed (p < 0.05) from the thresholds in the horizontal plane.

The contribution of auditory cortex to spatial information processing was explored behaviourally in adult ferrets by reversibly deactivating different cortical areas by subdural placement of a polymer that released the GABAA agonist muscimol over a period of weeks. The spatial extent and time course of cortical inactivation were determined electrophysiologically. Muscimol-Elvax was placed bilaterally over the anterior (AEG), middle (MEG) or posterior ectosylvian gyrus (PEG), so that different regions of the auditory cortex could be deactivated in different cases. Sound localization accuracy in the horizontal plane was assessed by measuring both the initial head orienting and approach-to-target responses made by the animals. Head orienting behaviour was unaffected by silencing any region of the auditory cortex, whereas the accuracy of approach-to-target responses to brief sounds (40 ms noise bursts) was reduced by muscimol-Elvax but not by drug-free implants. Modest but significant localization impairments were observed after deactivating the MEG, AEG or PEG, although the largest deficits were produced in animals in which the MEG, where the primary auditory fields are located, was silenced. We also examined experience-induced spatial plasticity by reversibly plugging one ear. In control animals, localization accuracy for both approach-to-target and head orienting responses was initially impaired by monaural occlusion, but recovered with training over the next few days. Deactivating any part of the auditory cortex resulted in less complete recovery than in controls, with the largest deficits observed after silencing the higher-level cortical areas in the AEG and PEG. Although suggesting that each region of auditory cortex contributes to spatial learning, differences in the localization deficits and degree of adaptation between groups imply a regional specialization in the processing of spatial information across the auditory cortex.